April 1, 2021

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This article was originally published as a chapter in the book “Design and Catastrophe: 51 Scientists Explore Evidence in Nature"

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The human eye is a fascinating example of optical and anatomical design.[1] Human vision has significant functional contributions ranging from tear film, cornea, lenticular optics, aqueous and vitreous humor, pupil, and retinal layers. Vision is possible due to the interaction of light with the optics of the eye and the efficient way an image is transferred to the retina, converted into information, and transmitted to the processing centers in the brain. At each component of the eye (tear film, cornea, iris, crystalline lens, and retina), there is less room for error for it to function efficiently. In this short essay I will describe some aspects of the eye design that give human vision its great accuracy and precision. The more I consider these aspects, the more I become convinced that we were indeed “fearfully and wonderfully made” (Ps. 139:14).

Tear Film

The most anterior refractive surface of the human eye is the pre-corneal tear film, which is only about 4 µm thick and has a refractive index of 1.34. The tear film lipid layer provides a smooth optical surface for the cornea that is vital for eye comfort and visual performance. After every blink, the tear film spreads over the cornea within one second and remains there for about four seconds. After five seconds, the ocular surface will start to experience tear breakup due to evaporation of the aqueous component of tears. If the eye doesn’t blink within about five seconds, tear film breakup will start to seriously degrade the image quality due to the large change in refractive index at the air–tear interface.

Corneal Transparency

The cornea contributes about two-thirds of the total optical power of the human eye (about 60 diopters). It consists of 80% stroma (fibers), which are filled with collagen fibrils about 30 nm in diameter with a spacing of 65 nm. The precise spacing of collagen fibrils produces a regularly arranged matrix that allows light to pass through without experiencing diffraction effects, thereby making the cornea transparent. A slight change in the spacing of the fibrils would cause the cornea to become opaque due to destructive interference of the light.

Lens Refractive Power

The crystalline lens contributes about one-third of the total static optical power of the eye. The lens helps in image formation on the retina and importantly provides for focusing at different target distances (accommodation). The lens has a biconvex form with its anterior surface over 1.5 times larger than the posterior surface. It has a thickness of about 3.6 mm (relaxed eye) and a diameter of about 9 mm. The crystalline lens is avascular and has no pigments. Thus, it absorbs little light, transmitting most of it to the retina. The refractive index (RI) of the crystalline lens varies radially from being largest at the center with an RI of 1.402 to smallest at the edge with an RI of 1.375. Such gradient index (GRIN) lenses are extremely difficult to manufacture or to find commercially available. Nevertheless, they are very commonly seen in nature, despite the specificity and complexity of the design. If humans were to possess a single refractive index lens instead of a GRIN one, we would be missing about eight diopters of optical power.

Duplex Retina

“The greater light to govern the day and the lesser light to govern the night” (Gen. 1:16). This statement fits well with the function of the retina. Based on the retinal architecture one could say “cone photoreceptors for day and rod photoreceptors for night.” The photopic (day vision) spectral sensitivity or luminous efficiency is about 550 nm, and scotopic (night vision) spectral sensitivity is about 507 nm. Our visual system shifts the peak sensitivity from day vision (550 nm) to night vision (507 nm), an adjustment called the Purkinje shift.

The retina contains about six million cones with a maximum at the fovea. The foveal pit, which is responsible for providing sharp central vision, is about thirty-five hundredths of a millimeter wide and is in the macular region of the retina. Cones are avascular, allowing for light to pass through without any loss. The cone system has exceptionally high spatial and temporal resolution due to the packing density of the receptors and their one-to-one connection with the ganglion cells in the fovea. The photopic (cones) system has superior chromatic and contrast sensitivity. On the other hand, there are about 120 million rod photoreceptors, and they are not present in the fovea. The rod photoreceptors are highly sensitive in dim light. The rod photoreceptor system plays a vital role in human night (scotopic) vision. This system has great ability to sum up signals (at ganglion cells) from multiple rod photoreceptors to achieve high sensitivity (spatial and temporal summation). For this reason, the absolute luminous efficiency of scotopic vision is three times higher than the photopic vision.

Retinal Adaptational Range

The luminance adaptational range of the human eye is from 1010 (sun) to 10-6 (light threshold/starlight) candelas/m2. The eye can adjust to about 10 billion units of retinal luminance, of which only one billion units are accounted for by the pupillary ability (dilation and constriction). The remainder is contributed by the retinal layers, especially the photoreceptors.

Functional Retina

The retina also includes horizontal cells, bipolar cells, amacrine cells, and ganglion cells. Without these cells, we would not be able to resolve fine details, leaving us with blurry images, lack of light modulation, inaccurate motion detection, and other perceptual challenges. Each retinal layer is vital for our visual performance.

If you look at the retinal architecture, it may seem counterintuitively arranged backward. The photoreceptor is located at the back of the retina next to the retinal pigment epithelium (RPE). This architecture is imperative as RPE replenishes nutrients to the receptor cells, absorbs light that was not absorbed by the photoreceptors, and keeps blood-borne pathogens from infecting the eye. So, after all, it is not backward, but cleverly designed.

NOTES

[1] For a more detailed treatment of the aspects of the human eye discussed in this essay, see D Atchison, G Smith. Optics of the human eye. Oxford (UK): Butterworth-Heinemann; 2000; SH Schwartz. Visual perception —a clinical orientation. 5th ed. New York: McGraw Hill; 2017; CW Oyster. The human eye. Sunderland (MA): Sinauer; 1999.

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Jesson Martin is an associate professor of optometry at Kentucky College of Optometry, University of Pikeville. He holds a PhD in Physics from the National Institute of Technology (Bharathidasan University) in India. He expanded his work as a postdoctoral research associate at Indiana University School of Optometry and at the Eye Institute of the Medical College of Wisconsin. His current research interests are in the prevention of myopia in children using smart devices. Some of his recent research presentations include Continuous Remote Monitoring Device-Eye Distance and Ambience Light in Children and Wavelength Optimization of the Retinal Image Quality. He serves as a reviewer for the Optical Society of America (OSA) journals and the Journal of Refractive Surgery.